Project Summary/Abstract The long-term goal of this study is to understand the function of ubp43 (usp18) in hematopoiesis. Hematopoiesis is a complex process of cell proliferation and differentiation. Regulation of hematopoiesis occurs at multiple levels including transcriptional regulation, signal transduction, and protein modification. We cloned a novel member of the deubiquitinating enzyme family, termed ubp43. Further studies have demonstrated that ubp43 is a protease that specifically removes the ubiquitin like protein ISG15 from protein conjugates. Like phosphorylation, ISGylation is a mechanism of protein modification. However, in contrast to phosphorylation, only a limited understanding exists of the mechanisms and consequences of ISGylation. Interferons strongly induce ubp43 expression. Ubp43 deficient mice have fundamental differences in their hematopoietic response to interferons. Furthermore, ubp43 deficient hematopoietic cells have significantly enhanced type I interferon signaling. Most importantly, ubp43 has deconjugating enzyme activity dependent and independent functions in interferon signal transduction. This proposal tests the hypothesis that ubp43 is crucial for hematopoiesis under stress conditions, such as viral and bacterial infections and genotoxic insults, by interacting with critical factors in hematopoiesis and in the JAK-STAT signaling pathway and by regulating the level of protein ISGylation. The studies proposed in Specific Aim #1 will characterize ubp43 enzyme activity in hematopoiesis. The studies proposed in Specific Aim #2 will investigate the molecular mechanism of inhibition of type I interferon signaling by ubp43. The studies proposed in Specific Aim #3 will study the role of ubp43 in hematopoietic stem cells and progenitors. The experiments proposed will address fundamental questions about ubp43 in hematopoiesis during stress responses, which may provide valuable insight into the treatment of blood related diseases. Project Narrative Blood cell formation is tightly regulated at different levels in the response to various stimulations. Ubp43 is highly increased in blood cells upon pathogen infection, interferon treatment, and other stresses. Understand of its role in blood cells may help to develop novel therapies to treat defects in blood cell generation and function, such as anemia and infections.